Universal Mobile Telecommunications System (UMTS) is a current standard for wireless radio telecommunication networks. Efforts are being directed to develop an evolved standard, UMTS Long Term Evolution (LTE), also known as E-UTRA (Evolved UMTS Terrestrial Radio Access).
In E-UTRA, in the downlink, orthogonal frequency division multiplexing (OFDM) is proposed. In OFDM, the sub-carriers are orthogonal to one another and each is modulated with a data stream. The downlink data modulation may be performed using QPSK, 16QAM or 64QAM.
In the uplink, single carrier frequency division multiple access (SC-FDMA) has been introduced, in which discrete Fourier transform (DFT) precoding is applied to spread OFDM signals across a single carrier. Orthogonality of in-cell users is achieved by the OFDM in the presence of frequency-selective fading. The DFT precoding allows an improved peak-to-average power ratio (PAPR) compared to what would be available if pure OFDM were to be implemented in the uplink. SC-FDMA may be used with frequency division duplex (FDD) or time division duplex (TDD) modulation in the uplink. Each sub-carrier used for transmission includes information for all the transmitted modulated signals, the input data stream being spread over them. Data transmission in the uplink is fully scheduled, involving transmission of scheduling request and scheduling information on the uplink. With the addition of higher-order modulation and AMC, large spectral efficiency is possible by scheduling users with favorable channel conditions.
To realize the large spectral efficiency of the SC-FDMA scheme, fast scheduling in time and frequency is necessary. This may add large amount of scheduling overhead both in the uplink and the downlink control channels, and may be inefficient to support large number of delay-sensitive, low data rate traffic flows. One possible approach for supporting low data rate traffic flows is to semi-statically allocate time-frequency zones, or interference avoidance schemes. This may result in low channel utilization.
Current 3GPP specifications relating to LTE are TS 36.211: Physical channels and modulation, which describes the multiple access scheme and TS 36.300: E-UTRAN overall description, Stage 2, which provides a description of MAC, including scheduling, and both these documents are incorporated herein by reference.